fix: handle tag@sha256:digest image refs in pull API

[Jasperhino]

Summary

• Fix image reference parsing in both coast-daemon and coast-service that truncated tag@sha256:digest refs at the colon inside sha256:..., producing invalid Docker pull requests (e.g., postgres:16.11-alpine3.23@sha256: with missing hash)
• When an image ref contains @, the full reference is now passed as from_image with an empty tag, which the Docker API handles correctly
• Adds 9 new tests covering the tag@sha256:digest format across coast-daemon, coast-service, and coast-docker

Context

coast build emits warnings when pre-caching images that use the tag@sha256:digest format in docker-compose.yml (e.g., postgres:16.11-alpine3.23@sha256:23e88eb...). The root cause is rfind(':') / rsplit_once(':') splitting on the wrong colon — the one inside sha256: — instead of the tag separator. This produces a truncated from_image and a bare hash as the tag in the Docker pull API call.

Images with just a tag (emqx/emqx:6.0.1) or just a name (atmoz/sftp) are unaffected.

Files changed

File	Change
coast-daemon/src/handlers/build/utils.rs	New parse_pull_image_ref() that handles @ before splitting on :
coast-service/src/handlers/build/images.rs	Updated parse_image_ref() with same early @ check
coast-docker/src/image_cache.rs	Tests only — this file already handled @ correctly

Test plan

• All 182 existing tests pass (cargo test -p coast-docker -p coast-core -p coast-service)
• New tests cover: digest-only (img@sha256:...), tag+digest (img:tag@sha256:...), registry+tag+digest (ghcr.io/org/app:v2.0@sha256:...), and registry-with-port (registry:5000/app:v1)
• Verify coast build no longer warns for digest-pinned images in docker-compose.yml

Made with Cursor

[jamiesunderland]

@Jasperhino do you have more to add to this or should I merge this?

[felipebn]

👋 Trying to use coasts and came across this issue too (filled #258). This fix would unblock our team. @Jasperhino would you mind if I rebase your branch and pick this up? Happy to keep your authorship via Co-Authored-By!